Multigrid tube amplifier circuit



Patented Jan. 10, 1950 2,494,317 MiiL'r'IGRID TUBE AMPLiFiER C'IRCUIT Arthur Mack Seybold, West Oran e, N. 1., as- Si'gnbr to Radio Corporation of America, a corporation of Delaware Application August 28, 1947, Serial No. 771,088

6 Claims.

In this application, I disclose an improved method of and means for operation of electron discharge tubes 'of the screen grid "or beam pentode type. My invention is of wide use in the radio and allied arts, the uses thereo'f being too numerous to include in this application. I have, however, disclosed herein one important application of myinvention. In this application, a tube is arranged and operated in accordance with 'my invention and "is used as a power amplifier such as might be used to amplify speech for modulating acarrier. Of course, this application per se of my invention is of wide use in the electronic -art'since power amplifiers are Widely used. F

The tube described is an RCA 807. Other screen grid tubesmay be used such as, for example, tubes of the 6L6 type. Tubes of this type have excellent class characteristics but have been used comparatively little in audio frequency modulator service. Perhaps this is because of the difiicu'lties encountered in providing the required regulation of control grid bias and screen grid voltages. The possibility "of using the 807 tube and others of the same type in class B audio service is intriguing. I ts unipotential cathode, its low price, it's small size and its ability to deliver a great deal of power at low plate voltage provided the impetus for a series of experiments, the primary purposes of which were to provide a circuit arrangement using thistube in a manner such as to bring out its ood reatures enumerated in part and to suppress or eliminate its undesirable characteristics. A more specific purpose of my invention is to eliminate the use of screen grid and bias supplies necessary for A132 operation of these tubes. Incarrying out my invention, work was started with screen grid tubes of the 6L6 type but later tests were made with the type 807 tube and these tests show that the system can be used with other multi grid tubes for numerous class B operations.

In describing my invention in detail, reference will be made to the attached drawings wherein Fig. l is a schematic circuit diagram showing the essential feature of my invention as used in a power amplifier.

Fig. 2 illustrates by a series of curves the progressive improvement of plate characteristics with addition of resistance in the #1 grid circuit.

Fig. 3 is a second family of curves illustrating the plate current voltage characteristics of tubes arranged and operated in accordance with my invention, while Fig. 4 is a basic diagram of a tube arranged and operated in accordance with my invention; such iii (Cl. ltd- 171 asm'ight be used to obtain the various characteristicsfillustrated "by the curves in Figs. 2 and 3. I In Fig. 1, the voltage to be amplified is fed to the leads indicated Input and amplified a preamplifier l of any approved type. A suitable amplifier for use here has been described in detail in a publication appearing in Ham Tips, May-June 1947, vol. VII, No. 2, entitled New modulator circuit utilizes 807s in class B with 'zero bias. The amplified output is fed to the primary winding of a transformer T2.

My invention is included in the next stage comprising tubes 12 and I4 of the pentode type having screen grid electrodes It and is coupled differentially by the secondary winding of the transformer T2. The control grid 20 of tube 12 is connected to the screen grid [5 by a resistor R while the control grid 22 of the tube M "is con nected to its screen grid l8 by a resistor R. The electrie'al center of the secondary winding of trans ormer T2 is tied to the indirectly heated cathodes and to ground and to the negative terminal of the plate source. Tubes [2 and M are otherwise substantially conventional in their arrangement and connection and the anodes are connected with the primary winding of an output transformer T3. 7

In the development of this circuit, plate 'faltnil'i'es were taken with various "values of resistance R between the #2 and #1 grids of tubes [2 and M. The series of curves shown in Fig. 2 illustrate the efie'ct of there'si'stance in the #1 grid circuit upon the shape of the plate family. The driving voltage designated is that which is applied directly to the #2 grid. Low values of resistance give poor knees bec'ausethe #1 grid is taking too much of the current. As the resistance is increased, the knee improves, until the optimum condition is reached at about 20,000 ohms. Higher values of resistance necessitate screen grid driving voltages that are higher than the type 807 tube ratings permit. In the curves of Figs. 2 and 3 E0 is voltage applied to the #2 grid. R is measured in ohms. EB is plate potential in volts. IB is plate current in milliamperes and I0 is current milliamperes to the #1 and #2 grids.

The 20,000 ohm value for R is a good compromise, and it canbe seen in Fig. 3 that when Ri20,000 ohms, an adequate plate family is produced. The grid current curve is depicted by a dotted line. With a "750 volt supply; a plate-toplate load or 6600 cum, and a driving source giving 554 peak volts grid-tO-gfid, 120 watts of audio are available: The power to drive the grids is greater than that needed for class A152; but the sacrifice is an easy one to make because a pushpull triode driver will easily furnish the 5.3 watts needed for the class B 807 grids.

The only important technical difference between 807s and regular zero bias class B triodes is in their eifective grid resistance. Whereas most of the high mu zero-bias triodes require lowvoltage high-current driving signals, the 807s take excitation at high voltage but with low current.

As stated above, any approvedamplifier may be used as the preamplifier 4. In Fig. 1, I have shown in rectangle 4 an amplifier I found satisfactory in operation. The voltage to be amplified is impressed on the primary Winding of a transformer Tl, a secondary winding of which is coupled in push-pull relation to the control grids of a pair of amplifier tubes 5 and 8. In the embodiment illustrated, these preamplifier tubes were of the 2A3 type with their cathodes heated by alternating current from a transformer T4, the secondary winding of which is connected to ground by a biasing resistor RI shunted by a large condenser Cl. The potential drop therein is applied to the center point of the secondary winding of transformer TI and operates the grids as desired. The anodes of the tubes 6 and 8 are coupled in push-pull relation by the primary winding of the transformer T2 and direct current potential for the anodes of the preamplifier stage is supplied to a point at the electrical center of this primary winding.

The 807 grids present a fairly constant load, applied continuously. A pair of 2A3s, which will give ten watts with a plate-to-plate loader 5,000 ohms, can be coupled to the 807s by a transformer T2 matching the 5,000 ohms of 2A3 plate load to the 7,100 ohms or equivalent grid resistance of the 807 class 13 stage.

The final item in the modulator is the output transformer T3. Any audio transformer designed to handle the required output will do, providing it will match the plate-to-plate impedance of the tubes l2 and [4 to the load which, in the case under discussion, might be the plate circuit of an RF amplifier operating class C.

As stated above, my invention was made during Work on power amplifiers. The principle involved, however, is much broader being the use of a resistor between the control grid and screen grid of a multigrid tube. The selection of the resistor will determine the operating characteristic of the tube and such selection is made with the use to which the tube is put in mind. For example, with a tube as arranged in Fig. 4, adjustment of the resistor B may be made to obtain various characteristics such as illustrated in Fig. 2. The range of resistor R values and magnitude of E6 of Fig. 2 are such as may be used with a type 807 tube. Different type tubes will require other values of resistance to obtain the optimum operating conditions for the job at hand. The variable potentials may represent control potentials of any type such as for tuning purposes or keying purposes and the like. The conductance of tube 12 and its current output changes with changes in the magnitude of the supplied screen grid potential in accordance with the tube characteristic as established by selection of R. The resistor L may represent the load and the current therethrough is controlled. For eX ample, a relay winding may replace L and then the relay may be controlled by the variable potential on'the #2 grid. Controlled voltage appears across L and may be used as desired.

What is claimed is:

1. In a voltage or current amplifier, an electron discharge tube having an anode, a cathode, a control grid and a screen grid, an alternating current input circuit, on which said voltages or currents to be amplified are impressed, coupled directly between said screen grid and said cathode, an alternating current output circuit coupled to said anode, and direct current circuits for the electrodes of said tube including a series resistor of appreciable value connecting said screen grid to said control grid and a source of direct current potential for maintaining said anode electro-positive relative to said cathode.

2. In a voltage or current amplifier, a pair of electron discharge tubes each having an anode, a cathode, a control grid and a screen grid, an alternating current circuit excited by voltages or current to be amplified coupled directly to the screen grids of said devices in push-pull relation, said push-pull coupling including a connection to the cathodes of the tubes, an alternating current output circuit coupled to the anodes of said tubes, and direct current circuits for the electrodes of said tubes including a series resistor of appreciable value connecting the screen grid of each tube to its control grid, there being a separate resistor for each tube, and a source of direct current potential for maintaining said anodes electro-positive relative to said cathodes.

3. An amplifier as recited in claim 2 wherein said resistors are of substantially equal value.

4. An electron discharge tube circuit, comprising an electron discharge tube having an anode, a cathode, a control grid, and a screen grid; a source of controlling voltage; means connecting said source directly to said screen grid; means connecting said control grid to said source and to said screen grid only through a resistor of appreciable value; and a utilization circuit connected between said anode and said cathode.

5. In an electrical energy repeater making use of an electron discharge tube which has an anode, a cathode, a control grid, and a screen grid; a circuit terminal for each tube electrode; means connecting the control grid terminal to the screen grid terminal only through a resistor of appreciable value; connections for applying a controlling voltage directly to the screen grid terminal; and means for connecting a utilization circuit between the anode and cathode terminals.

6. In connections for operating an electron discharge tube which has an anode, a cathode, a control grid, and a screen grid; a circuit terminal for each tube electrode; means connecting the control grid terminal to the screen grid terminal only through a resistor 0f appreciable value; connections for applying a variable controlling voltage directly to the screen grid terminal; and a utilization circuit connected between the anode and cathode terminals.

ARTHUR MACK SEYBOLD.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,745,369 Holden Feb. 4, 1930 2,023,506 Barton Dec. 10, 1935 2,084,181 Barton June 15, 1937 FOREIGN PATENTS Number Country Date 781,385 France May 15, 1935 

